This invention relates to certain improvements in oil shale retorting involving the solids-upflow, gas-downflow retorting technique, such as that described in U.S. Pat. No. 3,361,644. In that process crushed oil shale is fed upwardly through a vertical retort by means of reciprocating feed piston. The upwardly moving shale bed continuously exchanges heat with a downflowing, high specific heat, hydrocarbonaceous recycle gas introduced into the top of the retort at about 950.degree.-1200.degree. F. In the upper section of the retort (the pyrolysis zone), the hot recycle gas educes hydrogen and hydrocarbonaceous vapors from the shale. In the lower sections of the retort the oil shale is preheated to pyrolysis temperature by exchanging heat with the mixture of recycle gas and educed product vapors. Most of the heavier hydrocarbons condense in the lower portion of the retort, and are continually swept away from the hot pyrolysis zone and collected at the bottom of the retort as product oil. The uncondensed gas is then passed through external condensing and/or demisting means to obtain more product oil. The remaining gases are utilized as high Btu product gas, recycle gas as above described, and as fuel gas to preheat the recycle gas up to the above specified temperatures.
Retorting in the above manner generally requires a total heat input ranging between about 350,000 and 450,000 Btu's per ton of shale. This in turn entails recycling a large mass of recycle gas to provide the necessary heat carrying capacity. Conventionally, the process has been operated at atmospheric pressures or slightly below, entailing very large volumes of recycle gas, ranging up to about 30,000 SCF/ton of raw shale. It would be highly desirable to operate the retorting system at moderately elevated pressures of e.g., 10-50 psig, whereby the volume of recycle gas would be drastically reduced, thereby providing very significant savings in recycle gas compressor facilities and utilities, as well as improving heat exchange efficiencies. However, to operate at elevated pressures, means must be provided for introducing the shale into the retort and removing retorted shale therefrom without allowing the retort gases to depressure through the granular shale being introduced into, and removed from, the retorting zone. Conventional methods for achieving this objective involve the use of expensive and elaborate lock vessels, valves, star feeders, slide valves and the like. These devices are subject to failure through rapid wearing of moving parts, and tend to produce fines through crushing or abrading of the shale.
It has been found to be entirely feasible and reiable to maintain pressure in the retorting zone by providing relatively simple hydrostatic sealing arrangements through which solids are fed into and removed from the retort. In a typical arrangement, an elongated upwardly extending feed conduit is provided, communicating at its lower end with the cylinder in which the feed pump piston reciprocates, and with the liquid reservoir of oil which collects around the bottom of the retort. Raw shale is fed into the top of the feed conduit and retort pressure is allowed to establish a level of product oil in the feed conduit which may be, e.g., 5-50 feet higher than the oil level around the bottom of the retort. The raw shale gravitates downwardly as a compact bed through the oil seal and is intermittently fed into the feed cylinder and thence pumped upwardly into the retort.
If the oil forming the hydrostatic head in the shale feed conduit consists of full range retort product oil, a problem will sometimes be encountered when operating in cold climates. Full range shale oil normally has a pour point ranging between about 70.degree.-90.degree. F, while the shale entering the feed conduit may be at considerably lower temperatures. Under these circumstances, the shale oil will congeal and cause plugging or bridging of the feed conduit. This difficulty could of course be overcome by providing suitable heating devices for the feed conduit, but we have devised a much simpler and more economical solution to the problem. According to a preferred procedure, a low pour point relatively light fraction of the shale oil product is continuously recycled to the shale feed conduit so as to maintain an inventory of light oil therein, filling the conduit up to the level established by the retort pressure. The light oil flows downwardly into the retort feed mechanism and mingles with retort product oil from the lower disengaging section of the retort. In this manner, high pour point oil is essentially excluded from the shale feed conduit.
After passing upwardly through the retort, retorted shale at a temperature of about 900.degree.-1100.degree. F overflows the top of the retort and is gravitated downwardly through a suitable conduit into the top of a water quenching zone, in which a liquid level of water is maintained which is substantially below the level of the retorted shale therein. The bottom of the quenching zone communicates with an elongated, upwardly extending sealing leg in which a water level is maintained substantially higher than the water level in the quenching zone, sufficient to prevent the passage of retort gases through the sealing leg. A suitable solids conveying mechanism in the sealing leg carries spent shale upwardly and out of the sealing leg at a rate controlled to maintain the desired solids level in the quenching zone.
By operating the quench zone with a retorted shale level above the water level therein, superheated steam is produced which contacts the hot solids, and it is found that considerable gasification of the coke and/or hydrocarbonaceous deposits on the retorted shale takes place, producing substantial amounts of hydrogen and light hydrocarbons. The mixture of superheated steam and noncondensable gases generated in the water quenching zone is removed from this zone and the steam is then partially condensed in a quench tower by countercurrent contact with a circulating stream of water admitted to the top of the tower at a temperature between about 200.degree. and 240.degree. F. Substantial process heat is recovered from the circulating stream of water by conventional heat exchange means.
A complication in the operation of the quench tower arises from the fact that the superheated steam admitted thereto carries along with it a small amount of heavy hydrocarbons stripped from the retorted shale. If not removed from the system, these materials tend to accumulate and eventually form an asphaltic type deposit which can cause plugging problems and reduct heat exchange efficiency. It has been found that this problem can be solved very effectively by maintaining suitable temperature control of the overhead gas stream from the quench tower, so as to maintain sufficient steam flow overhead to strip out most of the hydrocarbons. This overhead gas stream is then further cooled to temperatures of about 100.degree.-200.degree. F to condense out the small amount of steam and hydrocarbons contained therein. The condensate is then separated from the noncondensable gases in a small secondary separator. The resulting oil and water phases are then separated, and the uncondensed gases comprising hydrogen and light hydrocarbons can be utilized as a low Btu fuel gas.